How do I get rid of ugly rings in toilet bowl?

Clorox toilet bowl cleaner and a toilet scrub brush.

I hate using bleach but have found it works so well for toilet cleaning that I just dump a decent amount it, let it sit overnight, and then scrub/flush in the AM.

pumice stone or a sanding sponge. neither of these should damage the toilet bowl. you will need to move the water out of the toilet. I have a toilet mop that does a great job doing that. when the water level is much lower use the pumice or sponge and scrub the hard water deposits away.

I second the Clorox toilet bowl cleaner, the one in the black squirt bottle. I use a magic eraser with this cleaner for stubborn stains.